Vibration detector for hard disk drives

ABSTRACT

A hard disk drive with a circuit that provides a control signal to a voice coil motor to move a head across a disk. The disk drive includes a circuit that provides a control signal to the voice coil motor to move the head. The circuit also has a state predictor and an inversion filter that compute a vibration estimate used to detect a vibration in the drive. When a threshold vibration is detected a vibration estimate is combined with the control signal to compensate for the vibration. The circuit detects the vibration without the need for an accelerometer or other sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detecting vibration within a hard diskdrive.

2. Background Information

Hard disk drives contain a plurality of magnetic heads that are coupledto rotating disks. The heads write and read information by magnetizingand sensing the magnetic fields of the disk surfaces. Each head isattached to a flexure arm to create a subassembly commonly referred toas a head gimbal assembly (“HGA”). The HGA's are suspended from anactuator arm. The actuator arm has a voice coil motor that can move theheads across the surfaces of the disks.

Information is typically stored in radial tracks that extend across thesurface of each disk. Each track is typically divided into a number ofsegments or sectors. The voice coil motor and actuator arm can move theheads to different tracks of the disks.

Disk drives typically employ a servo to maintain the heads on thecenters of the tracks. Deviations from the track center line may createerrors in the reading and/or writing of data. The drive may be subjectedto vibration. For example, disk drives have a spindle motor that rotatesthe disks relative to the heads. Rotation of the spindle motor maycreate a vibration that is transmitted through the drive.

Some disk drives contain an accelerometer that detects the presence ofvibration. The output of the accelerometer is fed to the servo systemwhich then compensates for the vibration. The inclusion of anaccelerometer increases the cost and complexity of mass producing diskdrives. It would be desirable to detect vibration without anaccelerometer.

BRIEF SUMMARY OF THE INVENTION

A hard disk drive with a circuit that provides a control signal to avoice coil motor to move a head across a disk. The disk drive includes acircuit that provides a control signal to the voice coil motor to movethe head. The circuit also includes a state predictor and an inversionfilter that compute a vibration estimate used to detect a vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an embodiment of a hard disk drive;

FIG. 2 is a schematic of an electrical circuit for the hard disk drive;

FIG. 3 is a schematic of a servo for the hard disk drive;

FIG. 4 is a graph showing an integrated vibration estimate signalcompared with a threshold;

FIG. 5 is a graph showing a position error signal with and without thevibration detection of the present invention.

DETAILED DESCRIPTION

Described is a hard disk drive with a circuit that provides a controlsignal to a voice coil motor to move a head across a disk. The diskdrive includes a circuit that provides a control signal to the voicecoil motor to move the head. The circuit also has a state predictor andan inversion filter that compute a vibration estimate used to detect avibration in the drive. When a threshold vibration is detected thevibration estimate is combined with the control signal to compensate forthe vibration. The circuit detects the vibration without the need for anaccelerometer or other sensor.

Referring to the drawings more particularly by reference numbers, FIG. 1shows an embodiment of a hard disk drive 10. The disk drive 10 mayinclude one or more magnetic disks 12 that are rotated by a spindlemotor 14. The spindle motor 14 may be mounted to a base plate 16. Thedisk drive 10 may further have a cover 18 that encloses the disks 12.

The disk drive 10 may include a plurality of heads 20 located adjacentto the disks 12. The heads 20 may have separate write and read elements.The write element magnetizes the disk 12 to write data. The read elementsenses the magnetic fields of the disks 12 to read data. By way ofexample, the read element may be constructed from a magneto-resistivematerial that has a resistance which varies linearly with changes inmagnetic flux.

Each head 20 may be gimbal mounted to a suspension arm 26 as part of ahead gimbal assembly (HGA). The suspension arms 26 are attached to anactuator arm 28 that is pivotally mounted to the base plate 16 by abearing assembly 30. A voice coil 32 is attached to the actuator arm 28.The voice coil 32 is coupled to a magnet assembly 34 to create a voicecoil motor (VCM) 36. Providing a current to the voice coil 32 willcreate a torque that swings the actuator arm 28 and moves the heads 20across the disks 12.

The hard disk drive 10 may include a printed circuit board assembly 38that includes one or more integrated circuits 40 coupled to a printedcircuit board 42. The printed circuit board 40 is coupled to the voicecoil 32, heads 20 and spindle motor 14 by wires (not shown).

FIG. 2 shows an electrical circuit 50 for reading and writing data ontothe disks 12. The circuit 50 may include a pre-amplifier circuit 52 thatis coupled to the heads 20. The pre-amplifier circuit 52 has a read datachannel 54 and a write data channel 56 that are connected to aread/write channel circuit 58. The pre-amplifier 52 also has aread/write enable gate 60 connected to a controller 64. Data can bewritten onto the disks 12, or read from the disks 12 by enabling theread/write enable gate 60.

The read/write channel circuit 58 is connected to a controller 64through read and write channels 66 and 68, respectively, and read andwrite gates 70 and 72, respectively. The read gate 70 is enabled whendata is to be read from the disks 12. The write gate 72 is enabled whenwriting data to the disks 12. The controller 64 may be a digital signalprocessor that operates in accordance with a software routine, includinga routine(s) to write and read data from the disks 12. The read/writechannel circuit 58 and controller 64 may also be connected to a motorcontrol circuit 74 which controls the voice coil motor 36 and spindlemotor 14 of the disk drive 10. The controller 64 may be connected to anon-volatile memory device 76. By way of example, the device 76 may be aread-only-memory (“ROM”) that contains instructions that are read by thecontroller 64.

Each sector of a disk track typically has servo bits A, B, C and D. Thecontroller 64 may operate a servo routine utilizing the servo bits toposition the head relative to the track. The head is moved in accordancewith a position error signal (“PES”). The PES reflects the differencebetween a target position and the actual position of the head.

FIG. 3 shows a schematic of a servo system 100 used to control theposition of the heads. The servo is typically performed by thecontroller 64. The servo system 100 includes a controller 102 thatcomputes a control signal u. The control signal u is provided to thevoice coil motor 36 to move the heads. The position of the heads x isprovided back to the controller 102. The introduction of vibration d inthe disk drive is shown schematically as being combined with the controlsignal u.

The head position x is also provided to a state predictor 104. Thepredictor 104 also receives the control signal u and computes apredicted position of the head at a next state x. The predicted state xis combined with the head position x by adder 106. The output of theadder 106 {tilde over (x)} is provided to an inversion filter 108. Theinversion filter computes a vibration estimate {circumflex over (d)}.

The vibration estimate {circumflex over (d)} can be filtered by a lowpass filter 110 and integrated by an integrator 112. The integratedfiltered vibration estimate {circumflex over (d)} is provided to acomparator 114. The comparator 114 compares the integrated vibrationestimate {circumflex over (d)} with a threshold. If the vibrationestimate exceeds the threshold the estimate is combined with the controlsignal u by adder 116. The inversion filter 108 and low pass filter 110can be designed in accordance with the following equations andderivations.

Neglecting the high-frequency flexible modes, the voice coil motor maybe described with have the following dynamic model:

{umlaut over (x)}={dot over (v)}=K _(a)(u+d)  (1)

The constant K_(a) represents the acceleration constant of the actuator.The variables x and v represent head position and head velocity,respectively. The variables u and d represent the control input and theexternal vibration disturbance, respectively. The discrete equivalent tothe continuous-time system can be described by:

x(n+1)=A _(p) x(n)+B _(p) u(n)+B _(p) d(n)

x(n)=C _(p) x(n)  (2)

State vector x, system matrix A, column vectors B, row vector C aredefined by:

$\begin{matrix}{{{x(n)} = {{\begin{bmatrix}{x(n)} \\{v(n)} \\{{u\left( {n - 1} \right)} + {d\left( {n - 1} \right)}}\end{bmatrix}\mspace{14mu} A_{p}} = \begin{bmatrix}1 & T_{s} & {{K_{a}\left( {T_{s} - T_{d}} \right)}^{2}/2} \\0 & 1 & {K_{a}\left( {T_{s} - T_{d}} \right)} \\0 & 0 & 0\end{bmatrix}}}{B_{p} = {{\begin{bmatrix}{{K_{a}\left( {{2T_{s}} - T_{d}} \right)}{T_{d}/2}} \\{K_{a}T_{d}} \\1\end{bmatrix}\mspace{14mu} C_{p}} = \begin{bmatrix}1 \\0 \\0\end{bmatrix}^{T}}}} & (3)\end{matrix}$

The constants T_(s) and T_(d) are sampling period and control delay,respectively.

It is desirable to have a state predictor 104 that predicts headposition from the control signal u and the measured head position x. Acurrent state estimator can be introduced, which responds quickly tounknown disturbances.

x (n+1)=A _(p) {circumflex over (x)}(n)+B _(p) u(n)  (4)

{circumflex over (x)}(n)= x (n)+L _(p) [x(n)−C _(p) x (n)]  (5)

x is the prediction vector and {circumflex over (x)} is an intermediatevector used for purposes of implementation. L_(p) is a feedback gainvector. It can be shown that unknown disturbance can be estimated from aprediction error. Combining equations (4) and (5) together and usingequation (2) yields the following prediction vector:

x (n+1)=A _(p) x (n)+B _(p) u(n)+A _(p) L _(p) C _(p) [x(n)− x (n)]  (6)

Defining the prediction error vector as {tilde over (x)}(n)=x(n)− x(n)and then subtracting equation (6) from equation (2), provides aprediction error dynamics equation:

{tilde over (x)}(n+1)=(A _(p) −A _(p) L _(p) C _(p)){tilde over(x)}(n)+B _(p) d(n)  (7)

A transfer function from disturbance d to a scalar prediction error{tilde over (x)}(n)=C_(p){tilde over (x)}(n) can be described as:

$\begin{matrix}{{H_{d\; 2{pe}}(z)} = {\frac{\overset{\sim}{x}(z)}{d(z)} = {{C_{p}\left\lbrack {{zI} - \left( {A_{p} - {A_{p}L_{p}C_{p}}} \right)} \right\rbrack}^{- 1}B_{p}}}} & (8)\end{matrix}$

The disturbance can be estimated from the prediction error {tilde over(x)}(z) and the inversion of the transfer function H_(d2pe)(z).

$\begin{matrix}{{\hat{d}(z)} = {\frac{1}{H_{d\; 2{pe}}(z)}{\overset{\sim}{x}(z)}}} & (9)\end{matrix}$

The system H_(d2pe)(z) may be a non-minimum phase system (has unstablezero) because the VCM plant C_(p)[zI−A_(p)]⁻¹B_(p) is a non-minimumphase system due to the effect of control delay. The system H_(d2pe)(z)is also a proper system. For example, the transfer function can bedefined as:

$\begin{matrix}\begin{matrix}{{H_{d\; 2{pe}}(z)} = \frac{C_{p}{{adj}\left( {{zI} - A_{p}} \right)}B_{p}}{\det \left( {{zI} - A_{p} + {A_{p}L_{p}C_{p}}} \right)}} \\{= \frac{{K_{0}\left( {z - \beta_{stable}} \right)}\left( {z - \beta_{unstable}} \right)}{z^{3} + {\alpha_{1}z^{2}} + {\alpha_{2}z} + \alpha_{3}}}\end{matrix} & (10)\end{matrix}$

where |β_(stable)|<1<|β_(unstable)|. The inversion 1/H_(d2pe)(z) isunstable and unrealizable. In order to find a stable and realizableinverse, a non-minimum phase system H_(d2pe)(z) is decomposed into aminimum phase system and an all pass system.

$\begin{matrix}{{{H_{d\; 2{pe}}(z)} = {{H_{\min}(z)}{H_{allpass}(z)}}}{{H_{\min}(z)} = \frac{{- \beta_{unstable}}{K_{0}\left( {z - \beta_{stable}} \right)}\left( {z - \frac{1}{\beta_{unstable}}} \right)}{z^{3} + {\alpha_{1}z^{2}} + {\alpha_{2}z} + \alpha_{3}}}{{H_{allpass}(z)} = \frac{- \left( {z - \beta_{unstable}} \right)}{\beta_{unstable}\left( {z - \frac{1}{\beta_{unstable}}} \right)}}} & (11)\end{matrix}$

The minimum phase system of two systems is inversed to create a stableinversion filter which takes the following form:

$\begin{matrix}\begin{matrix}{{H_{inv}(z)} = {\frac{1}{H_{\min}(z)}\frac{1 - a}{z - a}}} \\{= \frac{\left( {1 - a} \right)\left( {z^{3} + {\alpha_{1}z^{2}} + {\alpha_{2}z} + \alpha_{3}} \right)}{{- \beta_{unstable}}{K_{0}\left( {z - \beta_{stable}} \right)}\left( {z - \frac{1}{\beta_{unstable}}} \right)\left( {z - a} \right)}}\end{matrix} & (12)\end{matrix}$

The addition of the first order low pass filter (1−a)/(z−a) makes theinversion filter a realizable system. An estimate of disturbance d isdetermined by passing the prediction error through the inversion filter.

{circumflex over (d)}(z)=H _(inv)(z){tilde over (x)}(z)  (13)

The disturbance estimate may be corrupted by high-frequency noisebecause of VCM plant uncertainty including high-frequency flexiblemodes. External vibration is typically present in a low frequency area(below 900 Hz). In one embodiment, a 6^(th) order low pass filter isused to remove the unwanted high-frequency component.

$\begin{matrix}{{H_{lpg}(z)} = {\left( \frac{{c_{1}z} + d_{1}}{z^{2} + {a_{1}z} + b_{1}} \right)\left( \frac{{c_{2}z} + d_{2}}{z^{2} + {a_{2}z} + b_{2}} \right)\left( \frac{{c_{3}z} + d_{3}}{z^{2} + {a_{3}z} + b_{3}} \right)}} & (14)\end{matrix}$

FIG. 4 shows an example of a vibration estimate versus a thresholdvalue. The vibration estimate exceeds the threshold at about 60 Hz. Atthis point vibration is detected and the estimate {circumflex over (d)}is combined with the control signal u to compensate for the vibration.

FIG. 5 shows a reduction in the position error signal (PES) with thedetection technique described and shown versus a disk drive withoutvibration detection.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

1. A hard disk drive, comprising: a disk; a head coupled to said disk;an actuator arm coupled to said head; a voice coil motor coupled to saidactuator arm; and, a circuit that provides a control signal to saidvoice coil motor to move said head across said disk, said circuitincludes a state predictor and an inversion filter that compute avibration estimate used to detect a vibration, said circuit includes acomparator that compares said vibration estimate with a threshold andsaid vibration estimate is combined with said control signal if saidvibration estimate exceeds the threshold.
 2. The hard disk drive ofclaim 1, wherein said vibration estimate is combined with said controlsignal when the vibration is detected.
 3. (canceled)
 4. The hard diskdrive of claim 3, wherein said circuit includes an integrator thatintegrates said vibration estimate.
 5. The hard disk drive of claim 1,wherein said circuit includes a low pass filter that filters saidvibration estimate.
 6. The hard disk drive of claim 5, wherein said lowpass filter is a sixth order filter.
 7. The hard disk drive of claim 1,wherein said circuit includes a controller.
 8. A hard disk drive,comprising: a disk; a head coupled to said disk; an actuator arm coupledto said head; a voice coil motor coupled to said actuator arm; and,circuit means for providing a control signal to said voice coil motor tomove said head across said disk, and detecting a vibration.
 9. The harddisk drive of claim 8, wherein said circuit means provides a vibrationestimate that is combined with said output signal when the vibration isdetected.
 10. The hard disk drive of claim 9, wherein said circuit meanscompares said vibration estimate with a threshold and said vibrationestimate is combined with the control signal if said vibration estimateexceeds the threshold.
 11. The hard disk drive of claim 10, wherein saidcircuit means integrates said vibration estimate.
 12. The hard diskdrive of claim 9, wherein said circuit means low-pass filters saidvibration estimate.
 13. The hard disk drive of claim 12, wherein saidvibration estimate is filtered with a sixth order filter.
 14. The harddisk drive of claim 8, wherein said circuit means includes a controller.15. A method for detecting a vibration in a hard disk drive, comprising:detecting a head position; computing a prediction error from the headposition; computing a vibration estimate by passing the prediction errorthrough an inversion filter; and, determining the vibration from thevibration estimate by comparing the vibration estimate with a threshold.16. (canceled)
 17. The method of claim 16, further comprisingintegrating the vibration estimate before comparing the vibrationestimate with the threshold.
 18. The method of claim 16, furthercomprising filtering the vibration estimate before comparing thevibration estimate with the threshold.